I must have looked wasted. My wife was quite worried. She was ready to meet a friend for lunch but wouldn’t leave me until a modicum of life force had returned to my face.

I had just gotten back from a six mile run. It was the first time I had been out in hotter, more humid weather, and my route was without shade for significant stretches. I was sitting in my living room feeling faint, wondering how my circulatory system could have forgotten to pump blood to my brain.

As normal consciousness gradually returned, I resolved on two things for future training:

• always carry water
• get out early in the morning before it gets hot

These are two useful ideas. But only later did I realize a lesson even more important. I realized that I hadn’t begun to feel faint when I was running. My brain only became affected when I sat down.

The cool down

When you run, your circulatory system adapts in three ways:

• Your heart beats faster and stronger. It can shoot 4-5 times more blood through your system each minute than when you’re resting.
• Most of the extra blood flow goes to the muscles of your legs. The blood vessels in your legs dilate in order to suck up more oxygen where you need it most.
• The action of your leg muscles helps recycle blood back to the heart. The rhythmic muscle contractions adds perhaps an extra 20-25% kick to the action of your heart pump.

What happens if you stop running and immediately sit down?

The blood vessels in your legs are still wide open. It takes a few minutes for them to return to their baseline state.

Meanwhile, you’re not getting that extra 20-25% power boost from the action of your leg muscles.

Your brain doesn’t get enough blood. The color drains from your face, you looked wasted, and you’re about to faint.

What I need to do, when I finish a run, is keep moving! I have to walk for 5 or 10 minutes before I sit.

I’ve still got my water bottle. I still check the weather before I go out and plan a route that keeps me in the shade. But cooling down has been the key.

More info from Training Peaks.

___________________________________________

Deepen your body of knowledge

Do we really need to drink all that water?